1. Field of the Invention
This invention relates to a thermostatic expansion valve to be used as an expansion means for a refrigerating system comprising also a compressor, a condenser and an evaporator and using a refrigerant for heat exchange. More particularly, the invention relates to a thermostatic expansion valve which normally controls the extent of a valve opening to keep the degree of superheat at a predetermined level so that the efficiency of the evaporator is kept at a high level and makes the refrigerant flow into the evaporator at a predetermined flow rate when the evaporating temperature of the refrigerant in the evaporator is low.
2. Description of the Related Art
FIG. 1 schematically shows the configuration of a typical conventional refrigerating system used in an air conditioner. In this system, an evaporator 10, a compressor 12, a condenser 14, a reservoir 16 and a thermostatic expansion valve 18, which is used as the expansion means, are serially connected by a single duct line 20 in the above mentioned order and the thermostatic expansion valve 18 and the evaporator 10 are further connected by the duct line 20 to form a closed loop.
The refrigerant in the closed loop is vaporized in the evaporator 10 by exchanging heat with ambient air. The refrigerant is liquidized in the condenser 14 by exchanging heat with ambient air after it is compressed in the compressor 12, and reaches the thermostatic expansion valve 18 by way of the reservoir 16. The pressure of the compressed and liquidized refrigerant is reduced by the expansion valve 18 so that the refrigerant can easily evaporate in the evaporator 10.
The thermostatic expansion valve 18 has a thermal bulb 22 located at the outlet of the evaporator 10, and the actuator vapor contained in the thermal bulb 22 changes its state from vapor to liquid or vice versa in response to the temperature of the superheated vapor of the refrigerant at the outlet of the evaporator 10. Any changes in the pressure of the actuator vapor due to its vapor liquid transformation are applied to the upper surface of a diaphragm housed in a valve-body driving chamber which is mounted in the upper portion of the thermostatic expansion valve 18, so that the diaphragm is displaced in the valve-body driving chamber. Since a valve body is connected with the lower surface of the diaphragm, the displacement of the diaphragm changes the degree of the valve opening of the valve body.
When the difference between the evaporation temperature of the refrigerant contained in the evaporator 10 and the temperature of the superheated vapor of the refrigerant at the outlet of the evaporator 10 (hereinafter referred to as the degree of superheat) exceeds a predetermined value (hereinafter referred to as the degree of static superheat), the thermostatic expansion valve 18 displaces the valve body from its closure position, and after that, the expansion valve 18 controls the flow rate of the refrigerant flowing into the inlet of the evaporator 10 so as to make the degree of superheat always take a predetermined value larger than the degree of static superheat. When the degree of superheat takes the above described predetermined value, the evaporator 10 operates with an optimum efficiency; therefore the above described predetermined value of the degree of superheat is called the degree of working superheat.
On the other hand, however, there are other requirements in the refrigeration system used for airconditioning of an automobile.
That is, there are requirements for the refrigeration system to be so designed that the thermostatic expansion valve does not move the valve body to the closed position in order that the refrigerant is kept flowing into the evaporator 10, even in a case where the degree of superheat (signal) is small while the evaporation pressure (or the evaporation temperature) is low.
The above described requirements follow.
(1) In order to prevent the outer surface of the evaporator from being covered with frost (or being frozen) while the evaporation pressure and the evaporation temperature is low, a great amount of the refrigerant is needed to flow into the evaporator to keep the evaporator flooded with the refrigerant.
Covering the evaporator with frost impairs the working efficiency of the evaporator.
(2) In a case where a variable capacity compressor (a compressor which senses the evaporation pressure and controls its capacity to lower it when the evaporation pressure is low) is used, in order to prevent the action of the variable capacity compressor from being unstable while a heat load becomes small, a small constant amount of the refrigerant is needed to flow into the evaporator.
When the heat load becomes small, the thermostatic expansion valve moves its valve body between the opening position and the closed position with short intervals owing to the original function of the valve, so that the evaporation pressure signal sensed by the variable capacity compressor becomes unstable to make the action of the compressor unstable.
These requirements are proper in circumstances where the compressor will not be damaged even if the liquid back flow phenomenon is produced in the compressor since rotary-type compressors are widely used in the small-sized refrigeration systems for air conditioners of the automobiles.
The following inventions are prior art devices which are employed to realize the above described requirements.
In one of the prior art devices, a notch or a bleed port is provided in a valve seat.
In this prior art, a passage way for flowing a refrigerant at a predetermined flow rate (a function of a pressure difference between a high pressure side and a low pressure side) is provided in addition to the construction for the thermostatic expansion valve, so that the refrigerant can flow at the predetermined flow rate even when the thermostatic expansion valve is closed.
The above described prior art, has a disadvantage in that the original ability of the thermostatic expansion valve is impaired because the refrigerant is a given amount, not relating to the control of the thermostatic expansion valve, flows even in a case where the thermostatic expansion valve must function as originally intended.
Actually, with this prior art, the above described requirements are not fully satisfied.
Another prior art device is disclosed in Published Unexamined Japanese Utility Model Application No. 61-153875, in which a special expansion valve with an additional function is discussed.
The construction of this special expansion valve is schematically shown in FIG. 2.
The valve-body driving mechanism of this special expansion valve is constructed by two main portions, a first valve-driving mechanism 26 which is constituted by a combination of a thermal bulb 22 and a diaphragm 24 and operates in the same way as the valve-body driving mechanism of the above described conventional thermostatic expansion valve when the evaporation temperature is high and the evaporation pressure is also high, and a second valve-body driving mechanism 30 which is located between the diaphragm 24 of the first valve-body driving mechanism 26 and a valve body 28 and operates as a constant pressure expansion valve, which is different from the above described thermostatic expansion valve, when the evaporation temperature is low and the evaporation pressure is lower than a predetermined constant value.
The second valve-body driving mechanism 30 has a diaphragm 32 which is located nearer to the valve body 28 than the diaphragm 24 of the first valve-body driving mechanism 26 so that a constant pressure expansion chamber 34 is formed between the diaphragm 32 and the diaphragm 24 of the first valve-body driving mechanism 26. The constant pressure expansion chamber 34 contains an actuator vapor whose pressure is kept at a predetermined level and is provided with a force transmitting member 36 having its two opposite ends abutted on the diaphragm 24 and the diaphragm 32. Outside the constant pressure expansion chamber 34, a diaphragm carrier 38 abutts the diaphragm 32, the carrier 38 is connected with a spring carrier 42 for carrying a spring 40 designed to bias the valve body 28 toward the closure position, by a connecting rod 44.
When the evaporation temperature and the evaporation pressure are high in the evaporator 10 of the thermostatic expansion valve having a construction as described above, as in the same way as a conventional thermostatic expansion valve, the valve body 28 is driven by only the pressure of the actuator vapor contained in the thermal bulb 22 of the valve-body driving mechanism 26 because, the pressure of the actuator vapor in the thermal bulb 22 of the first valve-body driving mechanism 26 applied on the diaphragm 24 becomes larger than that of the actuator vapor in the constant pressure expansion chamber 34 of the second valve-body driving mechanism 30.
On the contrary, when the evaporation temperature is low and the evaporation pressure is lower than a constant value in the evaporator 10, the valve body 28 is driven by only the pressure of the actuator vapor in the constant pressure expansion chamber 34 of the second valve-body driving mechanism 30 because the pressure of the actuator vapor in the thermal bulb 22 of the first valve-body driving mechanism 26 applied on the diaphragm 24 becomes lower than the pressure of the actuator vapor in the constant pressure expansion chamber 34 of the second valve-body driving mechanism 30. That is to say, this special expansion valve does not act as the above described conventional normal thermostatic expansion valve which acts as a constant pressure expansion valve. Consequently, the valve body 28 is moved to its open position regardless of the value of the degree of superheat.
The above described special expansion valve is effective in principle for solving the above listed two problems in the above described refrigerating systems, but a number of problems as described below arise when it is manufactured.
Firstly, the force transmitting member 36 and the constant pressure expansion chamber 34 must be worked with a high precision in order to make the force transmitting member 36 faithfully respond to small changes in the pressure of the actuator vapor contained in the thermal bulb 22.
Secondly, the constant pressure expansion chamber 34 is formed to have a large volume in order to prevent the action of the valve body 28 driving in the second valve-body driving mechanism 30 from being influenced by the change in the volume of the constant pressure expansion chamber 34 that is caused by the displacement of the diaphragm 24 due to fluctuation of the pressure of the actuator vapor in the first valve-body driving mechanism 26.
Thirdly, a relatively large plane area is required for the diaphragm 24 in order to drive that valve body 28 in the first valve-body driving mechanism 26 because the pressure of the actuator vapor produced in the, first valve-body driving mechanism 26 is low. Since the diaphragm 24 having a large plane area increases the displacement of caused by fluctuations in the pressure of the actuator vapor in the first valve-body driving mechanism 26, the constant pressure expansion chamber 34 must be formed to have a large volume in order to solve the above described second problem. Therefore, the outer dimensions of the expansion valve of the above described known type become large.
Fourthly, the above described expansion valve uses two diaphragms which require troublesome mounting processes.